Why does rig work only on one side of the mesh?
Several conditions have to be fulfilled to make automatic vertex group weight mirroring work:


bone names are correctly formatted, usually suffixed by .L or .R, respectively.
vertex groups for both sides exist on your mesh - these may be empty, but must exist for the mirrored side too. First  LMB select the mesh, &#8679; Shift LMB select the armature > &#9096; CtrlP > Armature Deform > With Empty Groups.
vertex groups must be enabled on the Mirror modifier, but they already are by default
make sure the mirror modifier comes before the armature modifier